Medical X-ray imaging is quite sensitive to the attenuation of X-rays by the actual human object examined The dynamic range of the images often comprises overexposed and/or underexposed parts where no details can be recognized.
U.S. Pat. No. 5,625,665 discloses an X-ray apparatus provided with a filter for limiting the dynamic range of an X-ray image formed on an X-ray detector. The filter has filter elements including one or more capillary tubes, one end of which communicates with a reservoir with an X-ray absorbing liquid. The adhesion of the X-ray absorbing liquid to the inner side of the capillary tubes can be adjusted by means of an electric voltage applied to an electrically conductive layer provided on the inner side of the capillary tubes. The degree of filling of the capillary tubes with the X-ray absorbing liquid is adjusted by way of the electric voltage value. The X-ray absorption profile can be adjusted within a short period of time, for example within one second, by adjustment of the electric voltages applied to the capillary tubes. This is a versatile filter, but adapting it to an existing X-ray apparatus is expensive.
Some X-ray apparatuses comprise so called contour filters used for changing dynamic range locally and also decreasing X-ray load. These are shaped metallic sheets, different ones of which may come in a set. A selected one of the sheets is pushed in place to provide a given attenuation of the passing through X-rays. Positioning of contour filters is traditionally carried out by manual control of the user. This manual positioning is, however, is often not effective.
Due to the disadvantages of the prior art methods, especially manual positioning the contour filters, there is a need for providing a method which makes it possible to improve the quality and dynamic properties of X-ray medical images where a set of contour filters is available. There is also a need for automated selection and positioning of a contour filter without any change in the relevant contour filter unit of the X-ray apparatus.